Why does the same material looks brighter/darker on another object?
The problem seemed to come from the orientation of the UV unwrapped faces.

I didn't have the same textures, but replaced the missing 2 textures with concrete and stone wall textures to check. First i could not find the problem, as shading, normals and even custom normals did not help. Then i unwrapped the 2 parts from scratch and gave them more seams to have a clean unwrap, not the "Smart UV Project".

I then made sure the selected areas were given each the materials to have a similar result as was shown in the question:


Using a sun light to check the shadowing of the wall sections, the smaller one always seemed to be reacting exactly the opposite way then the big one. When i pointed the sun on the walls the smaller one became dark, and when i directed the light away it became bright.

As of that moment, considering that i tried all the other options mentioned, i had only one other idea: UV map.

I selected the UV map of the small corner wall and rotated it by 90° by selecting all with A and then pressing R followed by typing in 90 and confirmed it with Enter, the process shown below:


The result was this:


Also regarding the pixel density, i believe the easiest way would be to select all involved objects after they are unwrapped, go in UV Editing and select all elements A, to then use the "Average Islands Scale" function in the UV menu:


Happy Blending